One apparatus disclosed in U.S. Pat. No. 4,904,996 (the '996 patent) is known in the art for measuring physical data from electrical lines. However, it seems that the usefulness of the apparatus of the '996 patent may be somewhat limited, as it only measures physical data in the form of electromagnetic radiation. Furthermore, the apparatus of the '996 patent lacks means for steering the measuring device onto the electrical line in question. It does not seem to be probable that the device in the '996 patent may be able to replace the manual work that has hitherto been connected to maintenance work on electrical high voltage lines. As these must be provided with joints it is necessary to test these joints with certain time intervals, and to repair them if needed. For safety reasons, such manual work is normally done after the high voltage line has been taken from operation, that is to say been made dead electrically.